Sample size of 12 per group rule of thumb for a pilot study

TLDR

Clinical trial protocols require justified sample sizes, yet many pilot studies lack prior data to inform such calculations. The study recommends 12 participants per group for pilot trials, with the aim that the resulting data inform future study designs. The 12‑per‑group rule is justified by feasibility, precision of mean and variance estimates, and regulatory requirements. © 2005 John Wiley & Sons, Ltd.

Abstract

Abstract When designing a clinical trial an appropriate justification for the sample size should be provided in the protocol. However, there are a number of settings when undertaking a pilot trial when there is no prior information to base a sample size on. For such pilot studies the recommendation is a sample size of 12 per group. The justifications for this sample size are based on rationale about feasibility; precision about the mean and variance; and regulatory considerations. The context of the justifications are that future studies will use the information from the pilot in their design. Copyright © 2005 John Wiley & Sons, Ltd.